1. Field of the Invention
The present invention relates to a magnetic detection device that detects field intensity induced by a magnetic mobile object, for example, a magnetic detection device that detects a revolution or a rotation angle of a rotating body, and more particularly, to a magnetic detection device capable of detecting a moving direction of a magnetic mobile object.
2. Background Art
Regarding a magnetic detection device that detects field intensity induced by a magnetic mobile object, there have been several techniques for a method of detecting a moving direction of a magnetic mobile object depending on a signal outputted from a magnetic detection sensor. In a case where a sensor output signal is a digital signal (rectangular wave), a method of applying voltage modulation to a sensor output signal and a method of applying time modulation to a sensor output signal are available for detection of a moving direction of a magnetic mobile object.
One of techniques in the related art for applying voltage modulation to a sensor output signal is disclosed in Japanese Patent No. 3588044. According to this method, a magnetic detection sensor accommodates a magnet that generates a bias magnetic field, first and second magneto-resistance effect elements disposed oppositely to the magnetic mobile object and side by side in a moving direction thereof, and output circuits for the respective magneto-resistance effect elements. The magnetic detection sensor detects a change in magnetic field in response to a movement of the magnetic mobile object and sets at least either a high level or a low level of the output signal to different potential depending on the moving direction. Accordingly, a computer unit detects the moving direction by measuring the potential of the sensor output signal.
Meanwhile, according to the method of applying time modulation to a sensor output signal, output timing (time) of a magnetic detection sensor varies with a moving direction of the magnetic mobile object. For example, when the moving direction is the forward direction, a duration of the signal is set to a and when the moving direction is the inverse direction, a duration of the signal is set to b(≠a). Accordingly, a computer unit detects a moving direction by measuring the duration of the sensor output signal.
The present invention relates to a method of applying time modulation to a sensor output signal and techniques for this method in the related art are disclosed, for example, in JP-UM-A-57-14858 and JP-A-2001-88632.
A magnetic detection device in the related art will be described first with reference to FIG. 15 through FIG. 18B.
FIG. 18A and FIG. 18B are views showing the configuration of a magnetic circuit in the magnetic detection device in the related art. FIG. 18A is a perspective view and FIG. 18B is a front view. Referring to FIG. 18A and FIG. 18B, a magnet 3 generating a bias magnetic field is disposed on the bottom surface of a signal processing circuit chip 2 formed monolithically with magneto-electric transducer elements 1a and 1b. This assembly is oppositely moved closer to a magnetic mobile object 4, so that the magneto-electric transducer elements 1a and 1b detect a magnetic field distribution developed by the magnetic field generated by the magnet 3 with an increasing proximity and movements of the magnetic mobile object 4 and output the detected magnetic field distribution as electric resistance and a change thereof. In order to detect a moving direction of the magnetic mobile object 4, it is necessary for the magneto-electric transducer elements 1a and 1b to cause a change in electric resistance with a time delay, that is, a phase difference, in association with the movements. To this end, for example, the magneto-electric transducer elements 1a and 1b are disposed side by side in the moving direction of the magnetic mobile object 4.
FIG. 15 shows a signal processing circuit of the magnetic detection device. The magneto-electric transducer elements 1a and 1b form bridge circuits 10a and 10b, respectively. When electric resistances of the magneto-electric transducer elements 1a and 1b change in response to a movement of the magnetic mobile object 4, bridge signals a and b are obtained as voltage changes. The bridge signals a and b are compared with thresholds c and d in comparing circuits 11 and 12, respectively, and converted to rectangular wave signals e and f, respectively. The rectangular wave signals e and f are inputted into a moving direction determination circuit 13 and a moving direction signal g is obtained. The moving direction signal g together with the rectangular wave signal f is inputted into an output signal generation circuit 14 and processed therein. A sensor output signal i is thus obtained as an output.
FIG. 16 shows operation waveforms of the magnetic detection device. In the drawing, small letters a and b represent bridge signals of the magneto-electric transducer elements 1a and 1b, respectively, small letters c and d represent thresholds of the respective bridge signals, and small letters e, f, g, and i represent signals of respective portions in the signal processing circuit. The waveforms show an example where the moving direction of the magnetic mobile object switches from the forward direction to the inverse direction. There is a phase difference between the rectangular wave signals e and f converted from the bridge signals a and b, respectively, and a combination thereof varies with the moving direction of the magnetic mobile object. The moving direction signal g is in a low level and a high level when the moving direction of the magnetic mobile body is a forward direction and an inverse direction, respectively. Time modulation is applied to switching timing from the high level to the low level, which is a position signal of the sensor output signal i, in sync with the rectangular wave signal f depending on the moving direction of the magnetic mobile object in such a manner that a time of the low level becomes a period t1 and a period t2 when the moving direction is the forward direction and the inverse direction, respectively.
FIG. 17 shows the bridge signal b of the magneto-electric conversion element 1b and the threshold d thereof, the signals e, f, and g of the respective portions in the signal processing circuit, and the sensor output signal i (the bridge signal a and the threshold c are omitted). The waveforms show an example where the moving direction of the magnetic mobile object switches from the inverse direction to the forward direction.
When the magnetic mobile object moves, the bridge signal b varies in response to protrusions (1) and (2) of the magnetic mobile object. The bridge signal b thus crosses the threshold d at a time A and the rectangular wave signal f switches from the low level to the high level whereas the sensor output signal switches from the high level to the low level.
Thereafter, the sensor output signal i returns to the high level at a time C after an elapse of the period t2.
At a time B at which the sensor output signal i is in the low level, the bridge signal b crosses the threshold d again. The rectangular wave signal f thus switches from the high level to the low level. Being within the period t2, the sensor output signal i cannot output the position signal in sync with the signal f of the signal processing circuit.
The computer unit determines the moving direction by measuring a duration of the low level of the sensor output signal i. The position of the magnetic mobile object is detected from the switching of the sensor output signal i at the time A from the high level to the low level. Because the duration of the low level of the sensor output signal i at the time C is the period t2, it is determined that the moving direction of the magnetic mobile object is the inverse direction and the position thereof is the protrusion (1). Thereafter, switching of the sensor output signal i again from the high level to the low level is detected at a time D. Because the duration of the low level of the sensor output signal i at a time E is the period t1, it is determined that the moving direction of the magnetic mobile object is the forward direction and the position thereof is the protrusion (1) from the sensor output signal i at the time D. Hence, the protrusion (1) after the switching of the moving direction of the magnetic mobile object is detected with a delay.
The detection delay may possibly be accumulated during repetitive operations in the forward direction and the inverse direction of the moving direction. Hence, the magnetic detection device in the related art is not suitably used when it is necessary to detect the moving position exactly by detecting the moving direction.